Convert from vershok to X unit

Unit Definition (vershok)
Obsolete Russian weights and measures were used in Imperial Russia and after the Russian Revolution until they were replaced in the Soviet Union by a metric system in 1924.

Unit Definition (X unit)
The X-unit (symbol xu) is a unit of length formerly used to measure the wavelength of X-rays and gamma rays. The X-unit is approximately 1.0021×10-13 metre. Defined by the Swedish physicist Karl Manne Georg Siegbahn (1886-1978) in 1925, the X-unit could not at that time be measured directly; the definition was instead made in terms of the spacing between planes of the calcite crystals used in the measuring apparatus.